In recent years, secondary batteries which are extremely thin and small have been under active development as power storage devices for use in personal computers, or mobile terminals, such as mobile phones, or for use in video cameras, satellites, vehicles, and other devices. As packaging materials used for such batteries, attention is being given to laminated packaging materials formed of a multi-layer film (e.g. packaging materials having a laminate configuration of base material layer/first adhesive layer/metal foil layer/second adhesive layer/heat-sealing resin layer). Laminated packaging materials formed of a multi-layer film are superior to conventional metal cans used as containers for battery elements, in that the materials have light weight and high heat dissipation, and allow for free selection of shape.
Secondary batteries are manufactured, for example, by forming a recess, using cold molding, on a part of a secondary battery packaging material formed of a multi-layer film, accommodating battery contents (positive electrode, separator, negative electrode, electrolyte solution, and other elements) in the recess, folding back the remaining part of the packaging material where the recess is not formed, and hermetically heat-sealing the edge portions, with the electrode terminals extending from the positive and negative electrodes sandwiched between the edge portions (hereinafter, such a battery may also be referred to as single-sided battery). In recent years, other types of secondary batteries have been manufactured, being provided with a packaging material in which a recess is formed on both sides thereof and bonded together to thereby efficiently accommodate more contents, for enhancement of energy density (hereinafter, such a battery may also be referred to as double-sided battery). To obtain high energy density with such secondary batteries, a deeper recess is required to be formed. However, a deep recess is prone to cause pin holes or breakage at the edge portions or corner portions of the recess where the degree of stretching is high in molding using a die.
To reduce the occurrence of pin holes or breakage in deeply molding a recess (in deep drawing), PTL 1, for example, describes protecting a metal foil by using a film, such as a stretched polyamide film or stretched polyester film, as a base material layer of the secondary battery packaging material.